1. Field of the Invention
The present invention relates generally to a technical field of wireless communications and more specifically to apparatuses and methods which transmit and receive a multimedia-broadcast multicast service (MBMS) channel.
2. Description of the Related Art
In next-generation mobile communications systems in which video and data communications are primarily conducted, capabilities far surpassing those of the third generation mobile communications system (IMT-2000) are called for, necessitating a sufficient realization of broadband, and increased capacity and speed in communications. Therefore, various outdoor and indoor communications surroundings are envisaged. In downlink data transmissions, not only a unicast scheme, but also multicast and broadcast schemes are conducted. More specifically, an increasing importance has been placed on transmitting an MBMS channel in recent years. The MBMS channel includes multimedia information broadcast to a large number of specified or unspecified users, and may include voice, character, still and video image, and various other content. (See Non-patent document 1, for example, for trends in future communications system.)
On the other hand, in a wideband mobile communications system, effects of frequency-selective fading due to a multipath environment becomes prominent. Thus, an orthogonal-frequency division multiplexing (OFDM) scheme holds promise as a next-generation communications scheme. In the OFDM scheme, a guard interval portion is appended to an effective symbol portion including information to be transmitted to form a symbol, a plurality of which symbols are transmitted during a predetermined transmission time interval (TTI). Portions of information included in the effective symbol portion make up the guard interval portion. The guard interval portion is also called a cyclic prefix (CP) or an overhead.
On the other hand, unlike the unicast channel, the MBMS channel of the same contents is transmitted from multiple cells. In principle, the unicast channel is transmitted to a specified user from one cell.
In “Area 1”, as shown in FIG. 1, which includes three base stations BS1, BS2, and BS3, the same MBMS channel is transmitted. Such an area as described above may be called a MBMS area. Similarly, in “Area 2”, which includes three base stations BS11, BS12, and BS13, the same MBMS channel is transmitted. The MBMS channels transmitted in Area 1 and Area 2, which are generally different, may be the same intentionally or accidentally. A mobile terminal (more generally, user equipment (UE), which includes mobile and fixed terminals) receives the MBMS channel of the same contents, which is transmitted from multiple cells. The received MBMS channel forms a large number of incoming waves or paths depending on the length of a radio propagation path. If the delay difference of the incoming waves falls within the range of the guard interval due to the characteristics of the OFDM symbol, the multiple incoming waves as described above may be combined without intersymbol interference (soft-combined), making it possible to improve the receive quality as a result of path-diversity effects. Therefore, a guard-interval length for the MBMS channel is set longer than a guard-interval length for the unicast channel.
Now, when a unicast channel is to be transmitted to a certain user apparatus, a cell-specific scrambling code is used for pilot, control, and unicast channels. Based on the received pilot channel, a user apparatus performs channel estimation and other processes, channel compensation for the control and unicast channels, and the subsequent modulating. A scrambling code, which differs from cell to cell, may be used to distinguish, from an interference signal from another cell, a desired signal. However, if the unicast channel is merely replaced by the MBMS channel (if the scrambling code which differs from cell to cell is used for transmitting the MBMS channel), the user apparatus has to perform the process as described above while identifying a signal from a surrounding base station (specifically, a pilot channel), which is difficult. From such a point of view as described above, it has been proposed to separately provide, for the MBMS, a scrambling code which is common to multiple cells included in the MBMS area (a common scrambling code). More specifically, a cell-specific scrambled code-multiplied pilot channel (a specific pilot channel), and a pilot channel which is common to multiple cells within a MBMS area (a common pilot channel) are provided, with the specific pilot channel being used for channel compensating a unicast channel, and the common pilot channel for channel compensating an MBMS channel.    Non-patent document 1: Otsu, “A challenge for systems beyond IMT-2000-a wireless approach”, ITU Journal, Vol. 33, No. 3, pp. 26-30, Mar. 2003
As described above, for the MBMS channel, the same MBMS channel, which is transmitted from cells varying in distance from a user apparatus, are combined, so that the delay spread and the fluctuation in the frequency domain becomes larger than the same for the unicast channel. Thus, the channel estimation accuracy in the MBMS channel may deteriorate relative to the same for the unicast channel.
Moreover, unlike the unicast channel, the MBMS channel is transmitted to all user apparatuses using the same MCS (modulation and channel coding scheme), so that the MCS must be set to the worst user envisaged. For example, when a user apparatus is moving at high speed, the communications environment becomes worse. However, consideration must be given to ensure that the MBMS channel meets a required quality level even in such an environment as described above.